macOS segfault: copilot crashes with EXC_BAD_ACCESS (SIGSEGV)

## Summary
`copilot` crashes with a segmentation fault on macOS when invoked from terminal.

## Repro
1. Launch terminal (cmux in my case).
2. Run `copilot`.
3. Process exits with segmentation fault (`[1] 94078 segmentation fault copilot`).

## Expected
CLI starts normally.

## Actual
CLI crashes immediately with SIGSEGV.

## Crash report
- File: `~/Library/Logs/DiagnosticReports/copilot-2026-04-23-153812.ips`
- Incident ID: `7CE3EF49-27EA-4633-A900-D701E388DE69`
- Exception: `EXC_BAD_ACCESS` / `SIGSEGV`
- Subtype: `KERN_INVALID_ADDRESS at 0x000000000000000b`
- Faulting thread: `V8Worker`
- Parent PID recorded in crash report: `94078`

## Environment
- macOS: 26.4.1 (25E253)
- Hardware: Apple Silicon (`Mac15,3` from crash report)
- Terminal host process in report: `cmux`

## copilot version
```
GitHub Copilot CLI 1.0.35-6.
Run 'copilot update' to check for updates.
```

## Additional notes
- Crash report includes symbols pointing into V8 worker/concurrent marking paths.
- I can attach the full `.ips` report if needed.

## Environment snapshot

```
ProductName: macOS
ProductVersion: 26.4.1
BuildVersion: 25E253
Darwin Kevins-MacBook-Pro.local 25.4.0 Darwin Kernel Version 25.4.0: Thu Mar 19 19:32:59 PDT 2026; root:xnu-12377.101.15~1/RELEASE_ARM64_T8122 arm64
Captured at: 2026-04-23 15:42:58 -0700
```

Start by reproducing `copilot` from a terminal on Apple Silicon macOS 26.4.1 and review the reported `.ips` crash data, focusing on the V8Worker and concurrent-marking frames. The issue is resolved when the CLI starts normally without an EXC_BAD_ACCESS or SIGSEGV in the reported environment.

索引モデルが issue の本文から書いたものです。